1. Field of the Invention
The present invention relates, in general, to precursors useful in making fine powders, and, more particularly, methods to produce precursors, particularly organometallic precursors, and fine powders.
2. Relevant Background
Powders are used in numerous applications. They are the building blocks of electronic, telecommunication, electrical, magnetic, structural, optical, biomedical, chemical, thermal, and consumer goods. On-going market demands for smaller, faster, superior and more portable products have demanded miniaturization of numerous devices. This, in turn, demands miniaturization of the building blocks, i.e. the powders. Sub-micron and nano-engineered (or nanoscale, nanosize, ultrafine) powders, with a size 10 to 100 times smaller than conventional micron size powders, enable quality improvement and differentiation of product characteristics at scales currently unachievable by commercially available micron-sized powders.
Nanopowders in particular and sub-micron powders in general are a novel family of materials whose distinguishing feature is that their domain size is so small that size confinement effects become a significant determinant of the materials"" performance. Such confinement effects can, therefore, lead to a wide range of commercially important properties. Nanopowders, therefore, are an extraordinary opportunity for design, development and commercialization of a wide range of devices and products for various applications. Furthermore, since they represent a whole new family of material precursors where conventional coarse-grain physiochemical mechanisms are not applicable, these materials offer unique combination of properties that can enable novel and multifunctional components of unmatched performance. Yadav et al. in a co-pending and commonly assigned U.S. patent application Ser. No. 09/638,977 which along with the references contained therein are hereby incorporated by reference in full, teach some applications of sub-micron and nanoscale powders.
Some of the greatest challenges in the cost-effective production of powders involve controlling the size of the powders as well as controlling the composition of the powder. Precursor properties are significant contributors to these powder characteristics.
Precursors for nano-engineered powders are needed to manufacture superior quality nanomaterials cost-effectively and in volume. Precursors significantly impact the economics of a process and quality of products formed. A number of different high temperature processes based on different precursors have been proposed for the synthesis of nanoscale powders. For example, U.S. Pat. No. 5,514,349 (incorporated by reference herein), teaches the use of solid conducting electrode precursors to produce metal and ceramic powders. One difficulty with this approach is the cost and conductivity of the electrode. Furthermore, this process limits the ability to produce complex compositions because the composition of the product is directly dependent on the composition of the electrode. A wide variety of solid precursor electrodes are not readily available, and many desirable products do not have any corresponding electrode.
As another example, it is known to those in the art that halides such as titanium chlorides and gaseous metal-containing precursors such as diethyl aluminum and silane are precursors for powder production. These precursors can be used as precursors for high temperature processes to produce submicron and nanoscale powders. Similarly, U.S. Pat. No. 5,876,683 (incorporated herein) teaches the use of these and similar gaseous precursors to produce nanoscale powders.
These precursors create the challenge of post-treatment of byproducts such as chlorine, which can increase process complexity and cost. The product quality may also suffer because of chloride contamination. Another limitation is that processing equipment must be configured to handle the corrosive intermediates and byproducts, hence, the processing equipment tends to be expensive, require more frequent maintenance, and/or have short useful lifetimes.
Yet another limitation of such processes is the hazard and operability of the system as these precursors can undergo spontaneous reaction with other species involved in the process. Similarly, because many species spontaneously react with air or water, vapor which may be involved in the manufacturing process or simply present in the manufacturing facility, handling and use becomes both problematic and expensive. Finally, another limitation of these high temperature processes is the general need to gasify the feed before it is added to oxygen, which requires additional processing equipment, additional cost, and creates more opportunity for variability and contamination in the production process.
Another approach to precursor selection has been to use high molecular weight chelate-type polymeric precursors (e.g. see U.S. Pat. No. 5,958,361 herein incorporated by reference in full). These precursors have also been used in a high temperature process to produce simple and complex oxide powders. However, these precursors are expensive to produce and have secondary byproducts. The nitrogen or halides in these precursors or their equivalent face many of the same challenges as above. The high molecular weight correlates with to high viscosity which can affect the size distribution of the powder produced.
Other approaches involve feeding solid powders into a high temperature process in order to break them down to smaller sizes. In these approaches it may be difficult to control size distribution and significant agglomeration of the particles. Moreover, the variety of powders that can be produced is constrained by the availability of appropriate starting powders. To the extent the larger starting powders are produced by similar processes described above, this technique incorporates many of the limitations described above as well.
In general, processes available until now are limited by the choice of the precursor they utilize. There is a need for a process that utilizes low-cost, readily available precursors to produce high quality nanoscale powders. Moreover, there remains a need for precursors for powder production that are environmentally benign and require minimal pre-processing costs in high volume.
Briefly stated, the present invention involves the production and selection of precursors used to produce fine powders of oxides, carbides, nitrides, borides, chalcogenides, metals, and alloys. Methods for making fine powders using the selected precursor include selecting a precursor mixture wherein the mixture comprises at least one metal containing precursor, the metal containing precursor has an average molecular weight of less than 2000 g/mol of the metal, the metal containing precursor has a normal boiling point greater than 350K, and the viscosity of the precursor mixture is between 0.1 to 250 cP. The precursor mixture is processed under conditions that produce the fine powder from the precursor mixture. Fine powders produced are of size less than 100 microns, preferably less than 10 micron, more preferably less than 1 micron, and most preferably less than 100 nanometers. Methods for producing such precursors and powders in high volume, low-cost, and reproducible quality are described.